


Hush-Kit asked Brian Clegg, author of Inflight Science, those aviation questions we all want answered (but felt we should know already).

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What are contrails, and what causes them?”

The word ‘contrail’ is just a contraction of ‘condensation trail’ – it’s what we usually call a vapour trail in the UK, but condensation trail is more accurate. In effect they’re long thin artificial clouds. A cloud is just a collection of tiny water droplets and one of the waste products of burning aircraft fuel is water. There’s a jet of water vapour emerging from the back of a jet engine. Initially it’s invisible – you can see water when it’s a gas – but the air around it is very cold and it quickly condenses into droplets of water (or even ice crystals if it’s cold enough), just like a cloud. If contrails are forming you’ll get one from each engine, but because they aren’t visible until they are a little way behind the plane, and they soon merge, we often just see one. But only the plane is high enough. The reason we get those high ribbons of white, but nothing near an airport is that the air gets colder as you go higher, and you need to be over around 10,000 feet before it’s cool enough for a contrail to form.


I understand that there are conflicting views on the basic principle of how a wing works, what are they?”

At a basic level, there’s no problem. A wing works by redirecting the flow of air that goes past it as it cuts through the sky. But it gets complicated when you look at detail, because the physics of fluid flow (and air is just as much a fluid as is, say, water) is horribly messy. We’re usually taught that the lift that a wing gets, holding the plane up is due to something called the Bernoulli effect. The argument is something like this: the wing is shaped so it is further over the top surface than the bottom, which means that the air over the top has to go significantly faster to keep up with the air going over the bottom. Because the air is moving faster it thins out, reducing pressure above the wing compared with the pressure underneath – so the wing feels a pull upwards. The Bernoulli effect does exist, but this explanation of the lift being caused by the air going faster to keep up is rubbish – the air has no way of knowing what it needs to do to keep up. The reality is significantly more complex, and a much simpler way of looking at it is Newton’s third law that ever action has an equal and opposite reaction. The shape of the wing deflects the flow of the air in a downward direction. As it’s pushing down on the air, the air pushes up on the wing, producing lift.



The X-29 in flight.

What are the advantages of a forward-swept wing and why have they not caught on?”

It’s primarily a manoeuvrability versus stability argument. An aircraft with forward swept wings is more able to cut through the air efficiently, making it able to make sharper sudden turns. It also gives increased lift for the same area. But you pay for this with more instability on turns, as well as a tendency to bend the wing more than is the case with a conventional shape, which becomes more significant with large aircraft wings. What that has meant in practice is that forward-swept wings have been limited to fighter jets where manoeuvrability is arguably more important than stability, and even there, the relatively small benefits have often not been considered sufficient to balance out the downside.

What are the biggest misconceptions about aircraft?”

You’d need a whole book for that – my Inflight Science  covers a whole range, whether it’s things we get wrong or that are just an unexpected surprise. One would be what’s going on if an announcement is made about the crew putting the doors to automatic. This isn’t, as many think, engaging a lock. In fact, most of the time aircraft doors don’t need a lock. They open inwards first, and once the there is a significant difference between the cabin pressure and the outside air, it would be pretty much impossible to open the door against that pressure. Strictly speaking the announcement should be ‘chutes to automatic’ as what is being switched on is the automatic deployment of evacuation chutes. Another aspect of aircraft that is often misunderstood is why a tug is used to pull the aircraft away from the stand, rather than using the aircraft engines. It would be perfectly possible to manoeuvre on engines alone – aircraft usually do on landing. But to reverse away from the terminal would mean sending the jet blast straight towards all that plate glass, carrying with it any debris, which would be distinctly dangerous. Using a tug does also save on fuel, and Virgin Atlantic did plan some time ago to use tugs for the whole trip from terminal to runway. Unfortunately there were two problems with this. One was that airports typically didn’t have space at the end of the runway for detaching tugs and getting them out of the way. But more significantly, towing reduces the lifespan of the undercarriage and manufacturers made it clear that more towing would mean more expensive replacement schedules for the airlines.


Airliners have essentially looked the same since the Boeing 707, when will the next configuration become popular and what do you expect it to be?”

It’s not quite as straightforward as that. Certainly the narrow bodied aircraft have continued with a basic that is similar to the first hugely successful long range jet airliner, but wide bodied aircraft like the Boeing 747 and the Airbus A380 with their distinctive double decks do present a considerably different configuration. And we shouldn’t forget the entirely different Concorde, which failed for political reasons rather than commercial ones. For the future, there is no reason why basic, workaday aircraft would not continue with essentially similar formats. Significant change is liable to come with technological transformation. I’d suggest this is likely to have two possible directions. One is a return to supersonic flight. This may never happen, but if it does, looking at, for instance, the Airbus concept plans filed this August, we’d see a radically different design both in wing shape and body format, partly to help reduce the noise levels that plagued Concorde. The other is the move to low carbon flight. Aircraft fuel is a very efficient way to store energy and at the moment it would be impossible to have a battery powered airliner. In the future, though, we could be looking at designs to massively reduce fuel consumption in exchange, perhaps for slower flights – perhaps even a new generation of airships.


Conventional helicopters seem to struggle to get above 200mph, why is that?”

A traditional helicopter relies on tipping forwards to get forward momentum – but its rotors are still at an inefficient angle to get a huge amount of forward thrust, and the engines are always putting a lot of effort into keeping the aircraft up as there is limited lift available. The other problem is that as the aircraft moves forward there is more lift on the rotor blades when they are at the front of the aircraft and moving into the wind than there is at the rear, where they are moving away from the wind. The faster the helicopter goes, the more opportunity there is for ‘retreating blade stall’ where the helicopter rolls towards the blade that’s heading backwards. The main potential to get around this is either by rotating the rotors through 90 degrees, as in a VTOL aircraft, or to have a pair of rotors on the same axis rotating in opposite directions. This would work in theory, but is extremely difficult to manage in practice.


What are the most promising aeronautical technologies now in development?”

At their most visible, I think these are primarily approaches to deal with the two issues mentioned above: flying faster and flying greener. Perhaps the most dramatic flying faster development is a British one in the Skylon space plane, which is a plane that can be used to reach low Earth orbit. The key to its potential success is the radically different Sabre engines, which are air breathing as long as it is possible to do so before switching to rocket mode. This is a remarkable achievement, as the engines have have to drastically cool the air then compress it to an immense pressure all in a fraction of a second to be able work with the liquid hydrogen fuel. The benefit is to lose an equivalent conventional rocket’s need to carry around 250 tonnes more oxygen than Skylon. Flying greener will come both from greater efficiency and an eventual move to alternative fuels – the ideal would be a major battery breakthrough, which would require batteries to store at least 100 times as much energy per unit weight. And we are seeing some remarkable developments in battery technology. Finally I would stress the less obvious software side. Aircraft are immensely complex devices and the increasing computerisation and software capabilities could lead to anything from significantly reduced fuel consumption to more effective autopilots and transformed passenger communication capabilities.


China has announced development of a smart stealth skin that counter radars of any wavelength  what are your thought on this?”

Stealth is a combination of technologies designed to minimize the ability of an enemy to detect the presence of a piece of military hardware. This is both about confusing radar signals and reducing the emissions that the plane makes, for example the heat from its engines. Usually stealth is a pragmatic technology. If it’s not possible to make something entirely invisible, then the idea is to modify its appearance. So, for instance, stealth technology has been tested on tanks that makes them look like an ordinary car, and similarly on aircraft, when it’s not possible to be entirely invisible the aim is to look like a flight of birds. The radar can be fooled passively by either absorbing or scattering the radar waves, or actively by generating a confusing signal. To try get around existing stealth technology, extremely high frequency radar, which has better resolution but less range, is used. The new Chinese material is better at absorbing these frequencies than existing stealth absorbers. But this is only an incremental improvement, not a total breakthrough.


Where is the safest place to sit on an airliner?

Safety on an airliner is about two things – ability to survive and ability to get out. Making sure your seat has easy access to a fire exit is one obvious step – apart from the main doors there will be window emergency exits with wider seat spacing. Crew, when travelling as passengers, sometimes count the number rows to the exit row, so they can feel their way to it if the cabin is dark or smoke filled. On location, in analyses of crashes the safest seats are those behind the wing, then those over the wing, and worst right up the front. The safest seats of all aren’t generally available though – they are the crew seats. They have the key advantage of facing backwards, which makes survival in a crash significantly more likely. Airlines would love to have all the seats facing backwards, but when they have been available (the old Trident, for instance, had some backward-facing seats), passengers are reluctant to use them.


What happened to Flight MH370?”
Malaysia Airlines flight 370 disappeared over the Indian Ocean in March 2014. This 12-year-old Boeing 777 powered by Rolls Royce engines had 227 passengers and 12 crew onboard. The disappearance was abrupt after the plane had deviated from its flight path and considered a mystery. For many months no trace was found, but a small amount of debris was discovered at Reunion Island over a year later. Any answer to the question of what happened has to be speculative. The plane may well have flown until it ran out of fuel. This would seem most likely to have happened if the crew were incapacitated either intentionally or by a loss of oxygen. The oddity here is that the autopilot settings appear to have been changed to take the plane off the published flight plan, which seems to suggest an outcome that was not purely accidental. It is unlikely there will ever be a definitive answer to what happened to this aircraft.

 If an aircraft crashes – what are the hallmarks that it was downed by an onboard bomb?”

The recent disaster that befell the Russian Metrojet flight 9268 from Sharm el Sheikh to St Petersburg is now thought to have been caused by a bomb on board. There a number of indicators that investigators will use to discover the causes of a crash. An explosion will often result in mid-air break up of the fuselage, which would result in the debris being spread over a significantly greater area than a crash in which the plane was mostly intact before impact. There can also be identifiable damage to and traces of unburnt explosive on parts of the aircraft or passenger belongings that were situated near to the explosive charge, while the flight recorder can pick up distinctive sounds of the explosion before ceasing to function.




Follow my vapour trail on Twitter: @Hush_kit

Brian Clegg (www.brianclegg.net) is a science writer with over 30 books in print, including Inflight Science and Build Your Own Time Machine. His latest title, How Many Moons Does the Earth Have is a mind-stretching science quiz book.
For more on Brian’s books see www.brianclegg.net


  The Quantum Age explains the amazing science of quantum physics – but that’s just a beginning in exploring the way this mind boggling science is responsible for 30% of GDP, and provides fascinating stories from the anti-communist paranoia that accompanied the development of the laser to the remarkable world of the super cool. See www.brianclegg.net/quantumage.html


  1. Sundog

    As an Aero engineer, his explanation of lift is partly rubbish. He obviously doesn’t know anything about the Kutta-Jukowski theorem and the Kutta condition. There are various ways to look at how lift is generated and none is better than the other, it’s just a matter of the information one is seeking about the flow that determines which method one uses.

  2. Allan Hall

    Helicopters forward speed is limited by dysmmetry of lift, as the helicopter flies faster the airflow over the retreating blade ‘reverses’ until only the outer few feet of the ‘advancing’ blade is generating lift. In other words the faster a conventional co-axial helicopter flies forward the less lift the rotor blades generate. Which is why powerful helicopters have more blades – blade density.

  3. Mark G Vancil

    I prefer to explain lift this way: The atmosphere is a closed system like an aquarium. Any object introduced into it takes up space. Air is thus displaced more over the top of the wing, less underneath. As it moves forward more air is displaced above creating a vacuum. The wing moves in the direction of the lower pressure. Same as a jet engine creates thrust however it creates a vacuum in front of it toward which it is drawn. Bernoullies, lifties and draggies are forces which act on bodies moving in the atmosphere such as flaps. Bernoulli more in carburetors and aircraft a/c systems.

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